Porous polymer films that allow the flow of a fluid are classified, depending on pore sizes that the materials have, into mioroporous polymer films having a pore size less than 2 nm, mesoporus polymer films having a pure size of 2-50 nm, and macroporous polymer films having a pore size greater than 50 nm. Among them, in the case of mesoporous polymer films, the mesopores are so largo that they allow a fluid to flow freely therethrough and the films have a relatively large surface area where a fluid is in contact with a porous polymer film, and thus they have been given many attentions as materials for water treatment.
Methods that have been used to date for producing polymer films include a sintering method in which polymer powder is introduced to a mold, heating the polymer powder to a temperature slightly lower than its moiling point and sintering is carried out by using a pressurizer; a phase inversion method, such as a non-solvent induced phase inversion method, vapor phase induced phase inversion method and a thermally induced phase inversion method; an orientation method in which a crystalline polymer is drawn to impart porosity thereto; and a track etching method. However, the sintering method, phase inversion method and orientation method have a difficulty in controlling the pore size and density, and the track etching method allows formation of uniform and consistent pores but requires expensive equipment and a complicated manufacturing process.
Korean Patent Laid-Open No. 2010-0123290 (2010 Nov. 24) discloses a method for producing a porous polyimide film, including the steps of: growing gold nanowires (used as catalyst seeds) on a silicon substrate by using chemical vapor deposition (CVD); pouring a polyimide solution to the substrate on which the gold nanowires protrude; curing the substrate on which the polyimide solution is coated to form a film; and removing the nanowires incorporated to the polyimide film by selective etching. Although the method for producing a porous polyimide film provides a uniform pore size, the method requires a two-step etching process and an additional step of removing gold nanoparticles and should be carried out under the consideration of the thickness of the porous polyimide film as a final product. Further, the method uses gold, resulting in an increase in cost utilizes a CVD process requiring expensive equipment, and is not amenable to scale-up.
In addition, Korean Patent Laid-Open No. 2007-0099778 (2007 Oct. 10) discloses a method for producing a porous material, including the steps of: providing a porous template including a plurality of pores; forming nanostructures by using the template through a solid-liquid-solid (SLS) process or vapor-liquid-solid (VLS) process; removing the template; dispersing the resultant nanostructures to a new casting or precursor material; orienting the nanostructures in a predetermined direction; and removing the nanostructures by etching. Although the method for producing a porous material provides a uniform pore size, it has a difficult in carrying out the step of forming nanostructures by introducing a gas, requires expensive equipment, and uses a complicated process for producing nanostructures.
Therefore, a novel method that allows control of a uniform pore size and density merely by using a simple process is required.